He says the finding has implications for the evolution and ecology of sexuality and gender, and animal populations in general.

For the study, the team, which included German and US researchers, studied the sharpnose sandperch (Parapercis cylindrica) an Indo-Pacific fish that forms harem-like breeding societies on coral reefs of up to 10 females per male.

They created 48 'harems' consisting of one male and two females in an aquarium at the Lizard Island Research Station in North Queensland.

After 16 days the male was removed from 25 of these groupings to induce sex-change in the dominant female.

The activity levels and aggression of individual fish were measured repeatedly during the experiment before and after the induction of sex-change.

"Females and males often maximise their chances of reproduction and survivorship in different ways, using different behavioural strategies," says Walker.

"However, we found that behaviour expressed in the female phase is not independent of behaviour expressed in the male phase at the individual level."

Walker says the study shows that while aggression on average increased during the course of sex-change as females adopted the male role, relative individual behavioural characteristics of fish were maintained.

"Individuals that were relatively more active and aggressive females were found to become relatively more active and aggressive males," he says.

Walker says their finding points to a genetic limitation to the independent adaptation of female and male behaviour, and this could help to explain why diverse personality types are observed in nature and maintained through evolutionary time.

He adds the study also indicates the potential for cross-sex behavioural correlations to impose a cost on hermaphroditic behavioural strategies, and suggest a new hypothesis for the evolution of separate-sex species from a hermaphroditic ancestry.